The present invention relates to signal processing, and more particularly to speech encoding and decoding apparatus and methods.
EVRC (enhanced variable rate coding) provides speech coding at bit rates of 8.55 kbps (Full-rate), 4.0 kbps (Half-rate), and 0.8 kbps (⅛ rate); EVRC is widely used for second generation CDMA cell phone systems. G.729ab (a low complexity version of G.729) provides speech coding at a bit rate of 8 kbps and is popular for VoIP (voice over Internet protocol). EVRC and G.729ab both use digital speech sampled at 8 KHz and both have an ACELP (algebraic code excited linear prediction) structure. That is, both first perform LP analysis on a (sub)frame to find LP coefficients and convert them to line spectral pairs (LSPs) for quantization; next, both perform a pitch extraction plus an adaptive codebook search, quantized the pitch and adaptive codebook gain; then both search an algebraic fixed codebook to find an interleaved multiple pulse excitation vector and fixed codebook gain; and lastly quantize the parameters. However, EVRC and G.729ab are not directly compatible. Indeed, EVRC has frames of 160 samples partitioned into three subframes of 53, 53, and 54 samples; whereas, G.729ab has frames of 80 samples partitioned into two subframes of 40 samples.
Traditional transcoding from EVRC to G.729ab simply decodes input EVRC, synthesizes the speech, and then encodes the reconstructed speech with G.729ab for output; but this requires large computing power. And there is a demand for a lower complexity transcoding.
Transcodings between GSM (global system for mobile communication) and G.729 and between EVRC and AMR (adaptive multi rate) with direct parameter transformation rather than decoding followed by encoding have been suggested. For example, Tsai et al, GSM to G.729 Speech Transcoder, Proc. IEEE ICECS 485 (2001) and Lee et al, A Novel Transcoding Algorithm for AMR and EVRC Speech Codecs via Direct Parameter Transformation, Proc. IEEE ICASSP II-177 (2003).